The objectives of this record-linkage investigation are (10 to assemble a roster of patients who were diagnosed with ataxia-telangiectasia (AT); (2) to evaluate the occurance of cancer in these individuals; (3) to identify first degree relatives (parents, offspring, and siblings), and second degree relatives to the extent feasible (grandparents, aunts, uncles, and cousins) and link rosters of these relatives to cancer registry records to determine the occurrence of cancers; and (4) to obtain blood and tissue samples on patients and their relatives for molecular evaluation. A molecular evaluation will provide important knowledge on the role of genetic predisposition in the carcinogenic process.